1. Field of the Disclosure
The present invention relates to optical fire/smoke detectors.
2. Description of the Related Art
Fires can occur in a variety of ways. The two most common forms of fires are slow smouldering fires and fast flaming fires.
A smouldering fire is a slow, low-temperature, flameless form of combustion. These fires develop slowly and generate a significant amount of smoke which is easily detected by an optical smoke detector. Smouldering fires are typically initiated on upholstered furniture by weak heat sources such as cigarettes or an electrical short-circuit.
Fast flaming fires develop rapidly, typically generating black smoke and toxic fumes and leave little time for escape.
The characteristic temperature and heat released during smouldering (typically 600° C.) are low compared to those in a fast flaming fire (typically 1500° C.). Fast flaming fires propagate typically about ten times faster than smouldering fires. However, smouldering fires emit a high level of toxic gases such as carbon monoxide. These gases are highly inflammable and could later be ignited in the gas phase, triggering the transition to flaming combustion.
Both optical smoke alarms which use an infra-red emitter LED and ionisation type smoke alarms are used in the detection of both types of fires.
Optical type smoke alarms have an operational disadvantage when compared with ionisation type smoke alarms. They are relatively insensitive to black smoke created during fast-flaming fires. The optical method of smoke detection relies on light reflected off smoke as it enters the smoke alarm chamber. Black smoke absorbs light, rendering itself nearly invisible under optical sensing conditions. As a result, there is a significant time delay before the optical alarm is activated. Ionisation type alarms don't suffer from the same reliance on reflected light and therefore usually respond to fast flaming fires more quickly than optical type alarms, typically more than twice as fast.
However, ionisation alarms have the disadvantage that, as they contain radioactive isotopes in their sensors they are subject to regulations concerning their manufacture and disposal. These regulations depend upon the country but can place a considerable burden on the manufacturer.
The present invention seeks to provide an improved optical smoke alarm.